Question

Why molality is preferred over molarity in expressing the concentration of a solution?

Answer 1

For this problem, we have to first write the formula of both molarity and molality due to which we can determine on which criteria the molarity and molality depends and how it affects the solution.

Complete Step-By-Step Answer:
- In the given question change to explain the reason behind expressing the solution in terms of molality instead of molarity.
- As we know that molarity is the amount of substance which is directly proportional to the moles of the solute and inversely proportional to the volume of solution in litres.
- Whereas molality is defined as the ratio of moles of solute to the mass of the solvent in kilograms.
- As we know that volume of the substance is affected by the change in the temperature of the solution because with an increase in the temperature the volume increases.
- But, the mass of the solvent is independent of the temperature and does not change with temperature change.
- Also, the concentration of the substance is defined as the ratio of mass and volume.
- So, if the molarity will be used for expressing the concentration then it will not remain constant with the temperature change.

Therefore, molality is used for expressing the concentration of a solution.

Note: Colligative properties are considered as the physical properties of the solution such as elevation of boiling point, depression in freezing point, etc. Here, also the molality is used to express the concentration.